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© 2014 Carl Lund, all rights reserved A First Course on Kinetics and Reaction Engineering Class 21

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Where We’re Going Part I - Chemical Reactions Part II - Chemical Reaction Kinetics Part III - Chemical Reaction Engineering ‣ A. Ideal Reactors ‣ B. Perfectly Mixed Batch Reactors ‣ C. Continuous Flow Stirred Tank Reactors 21. Reaction Engineering of CSTRs 22. Analysis of Steady State CSTRs 23. Analysis of Transient CSTRs 24. Multiple Steady States in CSTRs ‣ D. Plug Flow Reactors ‣ E. Matching Reactors to Reactions Part IV - Non-Ideal Reactions and Reactors

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Reaction Engineering with CSTRs Typically CSTRs are designed to operate most of the time at steady state Transient operation occurs whenever a reactor variable is changed ‣ Start up and shut down are examples of transient operation Factors that favor CSTRs ‣ Liquid phase reaction ‣ Large quantities of reactant to be processed ‣ Exothermic reactions ‣ Reactions with “unusual” kinetics Reactant inhibited reactions Auto-catalytic reactions ‣ Cold feed and exothermic reaction (auto-thermal operation) Feed is heated due to being mixed directly into the hot reactor contents; no need for a separate heat exchanger Disadvantages ‣ For reactions with “typical” kinetics, the rate of reaction is low throughout the process Due to mixing, reactant concentration is low and product concentration is high Need larger reactor volume (compared to batch or plug flow reactor) ‣ Not well-suited to gas phase reactions because gases are hard to “stir.”

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Qualitative Analysis of CSTRs Steady state CSTRs are fundamentally different from batch reactors ‣ The composition and temperature change during the time that reaction occurs in a batch reactor The amount of time reaction occurs is controlled directly ‣ The composition and temperature are constant during the time that reaction occurs in a steady state CSTR The amount of time reaction occurs is controlled by changing the flow rate On average, the reaction occurs for a time equal to the space time, τ Qualitative analysis of CSTR ‣ Conversion, concentration, temperature and other profiles as a function of space time behave similar to profiles for batch reactors as a function of processing time ‣ When comparing to batch reactors at processing times equal to the CSTR space time Concentrations and temperature change during the time the fluid is reacting in a batch reactor Concentrations and temperature are constant during the time the fluid is reacting in a CSTR Their values are the final values; i. e. the reactant concentration is low and the product concentration is high In an adiabatic reactor, the temperature is the final value; higher for exothermic reactions and lower for endothermic reactions

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Questions?

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Activity 21.1 The handout for Activity 21.1 lists 10 problems, each involving a CSTR Read through each problem and ‣ Determine whether it calls for the analysis of a steady state CSTR or a transient CSTR ‣ If you decide a problem involves a transient analysis, justify your response by identifying at least one reactor variable that will change over time

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Activity 21.1 Question 1: steady state Question 2: transient ‣ The outlet cell mass, among other things, will vary over time Question 3: transient ‣ The outlet concentrations of all reagents will vary over time Question 4: steady state Question 5: steady state Question 6: transient ‣ The reactant concentrations leaving the reactor will vary over time Question 7: steady state ‣ The outlet concentration of Z will not vary with time Question 8: steady state Question 9: transient ‣ The outlet concentrations of reactants and products will vary over time Question 10: transient ‣ The outlet temperature will vary over time

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Perform a qualitative analysis of a CSTR within which an auto-catalytic reaction is taking place isothermally. Sketch the conversion versus the space time. Then qualitatively compare to the behavior for the same reaction taking place in a perfectly mixed batch reactor. Qualitative Analysis of a CSTR

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Qualitative Analysis Key Facts ‣ Reactor is isothermal, therefore no thermal effects ‣ Reaction is autocatalytic, therefore the rate increases as the concentration of product increases Qualitative Analysis of CSTR ‣ Conversion equals zero at space time of zero ‣ Conversion increases as the space time increases, so initial slope is positive ‣ Further increase in the space time increases the conversion Therefore the product concentration increases Therefore the rate increases Therefore, the change in the conversion becomes greater The plot of conversion versus space time will initially be concave up (increasing slope) ‣ That trend can’t continue for all space times; the conversion would go to infinity Eventually the depletion of reactant causes the rate to decrease with increasing space time Therefore, the rate passes through a maximum Therefore the conversion passes through an inflection point ‣ From that point onward, the conversion versus space time slope decreases (concave down) As the space time approaches infinity, the slope approaches zero and the conversion approaches its equilibrium value

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Comparison to a Batch Reactor At low processing times, the rate in the batch reactor will be small because it starts with no product present At an equal space time, the rate in the CSTR will be larger because for the whole time the fluid reacts, the product is present Therefore, the CSTR curve will lie above the batch reactor curve ‣ The batch reactor curve will display an inflection point, just as the CSTR does and for the same reason At smaller processing times, the effect of the increasing product concentration predominates and the rate increases At larger processing times, the effect of reactant depletion predominates and the rate decreases

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Where We’re Going Part I - Chemical Reactions Part II - Chemical Reaction Kinetics Part III - Chemical Reaction Engineering ‣ A. Ideal Reactors ‣ B. Perfectly Mixed Batch Reactors ‣ C. Continuous Flow Stirred Tank Reactors 21. Reaction Engineering of CSTRs 22. Analysis of Steady State CSTRs 23. Analysis of Transient CSTRs 24. Multiple Steady States in CSTRs ‣ D. Plug Flow Reactors ‣ E. Matching Reactors to Reactions Part IV - Non-Ideal Reactions and Reactors

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